assume cs:code,ds:data,ss:stack
; 数据段
data segment
  db 'abcdef'
  db 1, 2, 3, 4, 5, 6
data ends
; 栈段
stack segment
  dw 1,2,3,4,5,6,0,0
  dw 0,0,0,0,0,0,0,0
stack ends
; 代码段
code segment

start: mov ax, cs
  sub ax, 3
  mov ds, ax
  
  mov bx, 0
  mov cx, 6
  
lp:  mov ax, ds:[bx]
  and ax, 5F5FH
  mov ds:[bx], ax
  inc bx
  loop lp
  
  mov ax,4c00H
  int 21H
code ends

end start